International Journal of Geology, Earth & Environmental Sciences ISSN: 2277-2081 (Online) An Open Access, Online International Journal Available at http://www.cibtech.org/jgee.htm 2014 Vol. 4 (2) May-August, pp. 87-97/Chakraborty and Mukhopadhyay Research Article © Copyright 2014 | Centre for Info Bio Technology (CIBTech) 87 A COMPARATIVE STUDY ON THE NATURE OF CHANNEL CONFLUENCE DYNAMICS IN THE LOWER JALDHAKA RIVER SYSTEM, WEST BENGAL, INDIA *Subhankar Chakraborty and Sutapa Mukhopadhyay Department of Geography, Visva-Bharati, Santiniketan-731235, West Bengal, India *Author for Correspondence ABSTRACT River confluences are unique and most complex phenomena in a river network. They are unique as they possess numerous downstream effects upon the main stream while they are the complex nodal areas as processes across river confluences have merely been understood yet. The dynamics of confluence points in terms of their spatial locations and associated causative processes coupled with downstream morphological changes along the lower Jaldhaka River system has been studied systematically based on methods incorporating both field and geo-spatial technologies spanning for a period of 86 years (1928- 2014). A comparative analysis has been worked out which attempted to focus on the nature and processes of dynamicity along two different river confluences. Our observations have cleared it out that, while an upstream confluence tends much stability in recent era after being unstable earlier, the downstream one follows uncertainty over much of the considered timeframe. The causative factors such as avulsions, aggradations, reactivation of palaeo-channels, river capture, junction angle and control of structural arrangements have been observed and their roles were justified during the assessment of movements of the confluences both towards upstream and downstream directions. Additionally, the confluence effects on main stream morphology in terms of width adjustment, sinuosity indices and braiding tendencies have also been assessed. Keywords: Aggradation, Jaldhaka River, Junction Angle, River Capture, River Confluence, Spatio- Temporal Shift INTRODUCTION River confluences occupy significant position in a drainage network as they have immense effect towards downstream direction. Instead of having quite geomorphic and hydraulic significances, river confluences got very less attention by the scholars. However, there are quite a good number of researches that have been carried out particularly on flow character, flow structure and flow mixing, hydraulic geometry, bed morphology and channel scouring at channel confluences. The works of Mosley, 1976; Ashmore and Parker, 1983; Bristow and Best, 1993; Rhoads and Kenworthy, 1995; Benda et al., 2004; Boyer et al., 2006; Roy and Sinha, 2007; Lane et al., 2008 and Stevaux et al., 2009 etc. have set up some pioneering landmarks in the study of channel confluences. The meeting of two rivers, each of them having independent morphological nature, flow pattern and flow character and sediment discharge regimes creates complex riverine environment introducing both morphological and hydrological alterations. Such confluences play a major role in routing of the flow and sediment through the fluvial systems. In general, flow dynamics and mixing at confluences are highly variable both spatially and temporally and part of this variability is because of the momentum ratio between the two intersecting rivers and due to variations in the planform geometry and bed morphology of the junction (Boyer et al., 2006). Such confluences determine the availability of water and the dispersal of sediments along the downstream reaches (Roy and Sinha, 2007). In fact, River confluences are sites of drainage systems with complex hydraulic interactions provided by the integration of two different flows which constitutes an environment of “competition and interaction” with gradual dynamism in flow velocity, river discharge and structure, physical and chemical properties of water and channel morphology (Stevaux et al., 2009). From hydraulic perspectives, river confluences are active sites of occurrence of turbulence with convergent and divergent movements, resulting in upwelling and down welling of flows and formation of lateral vortex (Morisawa, 1968).